Glaucoma is an ocular disorder associated with elevated intraocular pressures (IOP) that are too high for normal ocular activity and may result in an irreversible loss of visual function. In a human eye for example, a fluid called the aqueous humor functions to maintain pressure in the eye at a range of about 10 to 21 mm Hg. The aqueous humor is constantly secreted by the ciliary body, and functions to keep the globe of the eye firm while providing nutrition for the lens and the cornea. The aqueous humor is drained away at the base of the iris via a series of channels known as the trabecular meshwork back into the bloodstream. When the aqueous humor drains properly, an appropriate fluid pressure is maintained in the eye. However, when these outflow channels become blocked, the pressure in the eye may increase due to built-up of the aqueous humor in the eye, and the circulation of blood to the optic nerve and other parts of the eye may be restricted. If the intraocular pressure remains elevated for prolonged time periods, the fibers of the optic nerve may atrophy and the retina may lose function, leading to glaucoma. If left untreated, glaucoma may eventually lead to blindness. Approximately 70 million people are affected world-wide, of whom 6.7 million suffer from bilateral blindness (Quigley H. A. et al., Br J Ophthalmol, 2006, 90, 262-7).
Presently, glaucoma is not curable by any available treatment method. However, various options are available to control and perhaps delay the progression of the disorder. State of the art methods to treat glaucoma include medicinal therapy and surgery. In medicinal therapy, eye drops containing a pharmaceutical agent, such as latanoprost and levobunolol, are usually prescribed to the patient for self-administration to the affected eye(s). The eye drops may work by improving outflow of aqueous humor from the eye and/or decrease production of aqueous humor by the ciliary body. However, this method may suffer from disadvantages such as undesirable side effects as a result of the medications and ineffective treatment due to poor compliance with medication administration procedures.
In some cases, medicinal therapy alone may not be sufficient to alleviate pressure built-up in the eye. Under these circumstances, surgery may be performed as a next step in glaucoma treatment. Examples of surgical methods for the treatment of glaucoma include trabeculectomy, laser surgery and implantation of glaucoma drainage devices.
In a trabeculectomy, a partial thickness scleral flap is made in the scleral wall of the eye and an opening is made under the flap to excise a section of trabecular meshwork. The scleral flap is then loosely sewn back into place so as to allow continuous drainage of the aqueous humor fluid. Post-operatively, the aqueous humor may flow out of the eye through the opening and may be absorbed through the bloodstream, thereby resulting in a lowered IOP. However, this method may cause excessive scarring around or over the flap, which could decrease the amount of, or prevent the flow of, aqueous humor fluid from the eye. Consequently, IOP may not be adequately lowered. In other cases, there may be excessive filtering of the aqueous humor fluid from the eye, a condition termed as hypotemy, which may result in an IOP value that is too low.
In laser surgery, thermal energy from a laser is applied to a number of spots in the trabecular meshwork, with the object to facilitate outflow of aqueous humor from the eye in order to lower the IOP. However, this method suffers from complications such as inflammation of the trabecular meshwork and tissue scarring.
In some cases, the doctor may recommend implanting a glaucoma drainage device (GDD) in the affected eye(s). Generally, a GDD is implanted when glaucoma cannot be controlled using medication, trabeculectomy or laser surgery. It may also be used in cases where trabeculectomy or laser surgery would likely fail. The GDD works to provide an avenue in which aqueous humor fluid can be drained from the eye so as to lower the IOP. State of the art GDD includes a tube that is attached to a plate, such as that for an Ahmed GDD or a Baerveldt® GDD. Generally, implanting the GDD includes excising the conjunctiva to expose the eye muscles, placing the plate of the GDD between or underneath the eye muscles, and fastening the plate to the underlying sclera with permanent sutures. The tube of the GDD is then cut to appropriate length and inserted into the anterior chamber of the eye. The conjunctiva is subsequently put into place over the plate to cover the tube.
Even though the success rate of GDD implantation is generally high, however, there still exist issues that may be encountered after implantation of the GDDs. These include a) an abnormally low initial IOP, which may occur within weeks from implantation, and b) an abnormally high late IOP, which may occur several months after implantation. Of these, an abnormally high IOP that occurs later on post operation is generally deemed to be more serious as it is believed to be responsible for device failure in the majority of patients.
An abnormally high late IOP may occur due to formation of a fibrous capsule around the GDD as part of a natural wound healing process after invasive surgery. The fibrous capsule may increase the flow resistance by at least partially obstructing the drainage of aqueous humor fluid from the eye, thereby resulting in a higher IOP value compared to the period of time before capsule formation. Current state of the art GDDs are not able to mitigate the increase in IOP resulting from fibrous capsule formation. As a result, further surgical intervention is required for glaucoma control.
In view of the above, there remains a need for an improved glaucoma drainage device.